﻿/*
* ==============================================================================
*
* Author: 王忠伟
* Created: 2017-01-19 15:51:37
* CLR Version: 4.0.30319.42000
* Version: 1.0.0
* 
* Description: EF拼接查询
*
* ==============================================================================
*/
using System;
using System.Collections.Generic;
using System.Linq;
using System.Linq.Expressions;
using System.Text;

namespace MyCore.DAL
{
    public static class ExpressionBuilder
    {

        //        var predicate = ExpressionBuilder.True<Gift>();
        //        predicate = predicate.And(w => w.DataState == DataStateEnum.正常);     
        //List<Gift> list = db.Gifts.Where(predicate);

        public static Expression<Func<T, bool>> True<T>() { return f => true; }
        public static Expression<Func<T, bool>> False<T>() { return f => false; }
        public static Expression<Func<T, bool>> And<T>(
            this Expression<Func<T, bool>> first,
            Expression<Func<T, bool>> second)
        {
            return first.AndAlso<T>(second, Expression.AndAlso);
        }

        public static Expression<Func<T, bool>> Or<T>(
            this Expression<Func<T, bool>> first,
            Expression<Func<T, bool>> second)
        {
            return first.AndAlso<T>(second, Expression.OrElse);
        }

        private static Expression<Func<T, bool>> AndAlso<T>(
        this Expression<Func<T, bool>> expr1,
        Expression<Func<T, bool>> expr2,
        Func<Expression, Expression, BinaryExpression> func)
        {
            var parameter = Expression.Parameter(typeof(T));

            var leftVisitor = new ReplaceExpressionVisitor(expr1.Parameters[0], parameter);
            var left = leftVisitor.Visit(expr1.Body);

            var rightVisitor = new ReplaceExpressionVisitor(expr2.Parameters[0], parameter);
            var right = rightVisitor.Visit(expr2.Body);

            return Expression.Lambda<Func<T, bool>>(
                func(left, right), parameter);
        }

        private class ReplaceExpressionVisitor
            : ExpressionVisitor
        {
            private readonly Expression _oldValue;
            private readonly Expression _newValue;

            public ReplaceExpressionVisitor(Expression oldValue, Expression newValue)
            {
                _oldValue = oldValue;
                _newValue = newValue;
            }

            public override Expression Visit(Expression node)
            {
                if (node == _oldValue)
                    return _newValue;
                return base.Visit(node);
            }
        }

    }
}
